The invention relates to ship propulsion systems of the type incorporating air actuated clutches, and more specifically to an improved control system which controls both the air supply to the pneumatically operated clutches and the throttle speed of the ship's prime mover.
One form of marine propulsion system employs ahead and astern air actuated clutches for connecting the prime mover to a reversing reduction gear unit for each propeller. In an air actuated clutch, the clutch is engaged by the inflation of an inflatable rubber and fabric air gland bonded to an outer steel rim. Friction lining on the inner surface of the gland engages a cylindrical clutch drum when the gland is inflated. When the gland is fully deflated there is no clutch engagement, and when the gland is fully inflated there is conplete clutch engagement. Between these two extremes the degree of clutch engagement corresponds to the amount of inflation of the gland. In certain propulsion systems the degree of clutch engagement is controlled so that a controlled slip of the clutch is permitted. This allows very low propeller shaft speed; lower than that which would be accomplished at engine idle with full clutch engagement. This is particularly advantageous for maneuvering the ship when docking or traveling in a congested area.
In my earlier U.S. Pat. No. 3,727,737 issued Apr. 17, 1973, for "Pressure Modulating System for Reversing Clutches and Throttle Control", I disclosed a pneumatic clutch control assembly for a ship's propulsion system that was sequentially operated to regulate the inflation of ahead and astern air inflatable clutches and to also control the prime mover speed. The control assembly was actuated by a single throttle lever located on a pilot house control stand. The single lever, if pivoted in either direction from the neutral, supplied air to a selector valve which selected one or the other of the ahead and astern clutches. Thereafter, and up to a first control pressure, air pressure proportional to the position of the lever away from neutral fed through a first valve to the clutch and thereby begin inflating the selected clutch. During this time the engine would remain at idle speed. After a first control pressure was reached, the first valve was piloted and connected a second path for air to the clutch. This second path had provision for an initial programmed rate of feed of air to the clutch through a choke valve so as to softly inflate the clutch. Upon reaching a second higher control pressure, full supply air pressure was connected to the clutch. After the first control pressure was reached, the continued inflation of the clutches was not dependent upon the position of the throttle lever.
When the air pressure within the clutch rose to a predetermined level, the control of my earlier patent piloted a governor valve which, in effect, connected the throttle lever control to the speed governor of the engine so that the pressure supplied to the governor directly corresponded to the position of the throttle lever and the speed could be controlled by movement of the throttle lever.
Accordingly, my earlier control assembly provided for single lever control of both direction and speed. Forward movement of the lever provided forward rotation of a propeller at a speed which increased with handle travel away from neutral. Backward movement of the handle provided astern rotation of the propeller with speed increasing as the handle was moved farther from neutral. The center position provided a neutral setting in which the engine was disconnected from the propeller and no power was transmitted, although the engine continued to idle. The throttle lever determined only the final operating speed and direction and all intermediate steps of clutch engagement and inflation and engine governor speed were handled automatically by the control system.
In my earlier control, once the governor valve controlling the engine governor was actuated by clutch pressure above a certain level, that valve remained actuated until the clutch pressure fell to below that level. This was true regardless of the engine speed once the valve had been actuated. Where the air supply to the system lost pressure for any reason, such as a malfunction in supply, the lower supply pressure would be reflected directly in lower pressure within the clutch. This lower pressure in the clutch might be insufficient to maintain full engagement of the clutch at high engine speed with the result that the clutch could slip at high engine speed.
I have provided by the present invention an improved control system which prevents accidental damage to the clutch due to slippage resulting from reduced air supply pressure. In accordance with my present invention, I have provided a control system which maintains a fixed relationship between the governor signal controlling the speed of the engine and the clutch internal air pressure such that if a normally operating system should begin to lose air pressure, the governor signal will be gradually cut back as the pressure falls thereby automatically reducing engine speed. This will prevent clutch slip, and the lower speed should alert the vessel's operator to the existence of a problem in the air supply.